Modeling Non-Newtonian Debris Flows In Hec-Ras: Two Diverse Applications

Debris flows are a dangerous and destructive geologic process that can transport large volumes of sediment, boulders, trees, and other debris. Debris flow hazards have long been recognized as an issue in mountainous regions and areas with developments on alluvial fans. Awareness for post-wildfire impacts to watersheds and resulting debris flows has been increasing. HEC-RAS version 6.0 included the ability to model non-Newtonian debris flows, which is more appropriate for high-concentration events. This paper highlights two diverse applications: 1) a community planning study in Ouray, Colorado and 2) an emergency post-wildfire inundation mapping effort in Mapleton, Utah.

Ouray, Colorado is situated in the steep and highly erosive San Juan sub-range of the Rocky Mountains. Debris flows have been recorded since the 1870s when the area was developed for mining. Several of the drainages around Ouray, including Corbett Creek, experience frequent debris flows which pose a threat to residential areas and infrastructure (roads, utilities, etc.). A road crossing that passes over Corbett Creek and serves as the secondary evacuation route for the city has been damaged or destroyed by debris flows twelve times in the last fifteen years. The County of Ouray required a more sustainable solution, and non-Newtonian debris flow modeling in HEC-RAS was utilized to evaluate the existing conditions and propose alternatives for the Corbett Creek crossing. Parameters were calibrated to match the model results to field observations of tree scour marks and sediment deposition within and outside the channel. Modeling with non-Newtonian parameters resulted in floodplains that matched the depth and extents of previous events. This approach provided benefit over modeling with Newtonian parameters and was used to test alternatives and propose a culvert sized to pass debris flows.

Non-Newtonian modeling in HEC-RAS was also used in an emergency application to rapidly develop inundation maps for the City of Mapleton, Utah following a wildfire. In October 2020, the Ether Hollow fire burned 849 acres in a steep drainage upstream of a residential area which resulted in a high likelihood of a debris flow event occurring, as predicted by the USGS. Debris yield estimates from the USGS and a simplified hydrograph produced with Wildcat5 software from the US Forest Service were used to develop a HEC-RAS, 2D, non-Newtonian debris flow model and inundation maps in two weeks. These maps were used to inform emergency personnel which homes to put on evacuation notice and to evaluate placement of flood barriers. An evacuation plan was successfully implemented during the summer of 2021 when high intensity rainfall resulted in a minor debris flow and evacuations kept community members out of harm's way.